Complex microelectronic devices such as modern semiconductor chips require numerous connections to other electronic components. For example, a complex processor chip may require hundreds of connections to external devices.
Typically, microelectronic components such as chips are mounted on substrates such as circuit panels having electrical contacts, and the contacts on the chip are electrically connected to the contacts of the substrate. The substrate may be a circuit panel with internal circuitry connected to the contacts. The substrate may be adapted to accommodate other components, including additional chips. Also, the substrate may have pins or other connectors adapted to connect the contacts or internal circuitry of the substrate to a larger assembly, thereby connecting the chip to the larger assembly.
Connections between microelectronic elements and substrates must meet several demanding and often conflicting requirements. They must provide reliable, low-impedance electrical interconnections. They must also withstand stresses caused by thermal effects during manufacturing processes such as soldering. Other thermal effects occur during operation of the device. As the system operates, it evolves heat and the components of the system, including the chip and the substrate expand. When operation ceases, the components cool and contract. When the assembly is heated and cooled during manufacture or in operation, the chip and the substrate expand and contract at different rates, so that portions of the chip and substrate move relative to one another. Also, the chirp and the substrate can warp as they are heated and cooled, causing further movement of the chip relative to the substrate. These and other effects cause repeated strain on electrical elements connecting the chip and the substrate. The interconnection system should withstand repeated thermal cycling without breakage of the electrical connections. The interconnection system should provide a compact assembly, and should be suitable for use with components having closely-spaced contacts. Moreover, the interconnection should be economical.
Various solutions have been proposed to meet these needs. In particular, as disclosed in U.S. Pat. Nos. 5,148,265; 5,148,266; 5,455,390 and in International Publication WO 96/02068, flexible leads may be provided between the contacts on a chip or other microelectronic element and the contact pads of a substrate. According to preferred embodiments taught in these documents, a compliant layer, such as an elastomer or a gel may be provided between the chip and the substrate. Flexible leads connecting the chip and substrate may extend through the compliant layer. In these preferred arrangements, the chip is mechanically decoupled from the substrate, so that the chip and substrate can expand and move independently of one another without excessive stress on the electrical connections between the chip contacts and the contact pads of the substrate. Moreover, the assemblies disposed in these patents and publications meet the other requirements discussed above. In certain preferred embodiments according to these documents, the chip and the interconnections to the substrate can occupy an area of the substrate about the same size as the chip itself.
Nonetheless, still further improvement would be desirable. For example, it would be desirable to provide additional connection components and methods which provide effective mechanical decoupling and high resistance to thermally induced stresses, while also providing low cost and high reliability.